In 1953 Harold Clay
ton Urey, the discoverer of heavy water, invited Stanley Miller,
one of his graduate students, to make an unusual experiment. In a hermetically
sealed and sterilized glass vessel Miller mixed the gases methane, ammonia and
hydrogen (which Urey, following the ideas of the Russian A. I. Oparin, believed were
constituents of Earth's primitive atmosphere) and water vapour. An electrical
discharge was passed through the circulating mixture for seven days. The result was
astounding: the reaction which had taken place produced organic substances:
ammo-acids, complex compounds which are integral constituents of proteins and
present in all living matter.
As the result of his
experiment Miller had obtained scarcely more than a milligram of
amino-acids; and since these are not living matter, the American chemist had not
succeeded in creating life in a glass vessel. Nevertheless his success in
synthesizing organic acids was full of implications: had he come near to creating the
conditions which had given rise to the first form of semi-living organisms in a
primeval sea a thousand or two thousand million years ago?
What were those organisms
like? Possibly—some biologists would say
probably—they resembled viruses such as are known today. Viruses, the cause of
many diseases, occupy a position between living and non-living matter. They are too
small to be seen except with an electron microscope, and it was the brilliant work of
an American biochemist, W. M. Stanley, in 1935 that proved them to consist of a
speck of nucleic acid cloaked in protein. All living cells are made up of these two
groups of chemical substances.
Viruses are incapable
of independent existence, except briefly, and survive only as
parasites in the cells of higher organisms; they do not breathe or show organic
development. Biologists therefore do not class them as 'living'. There are, however,
similar microorganisms, bacteriophage viruses, which can move independently. It is
tempting to assume that the first 'living' organisms resembled the free-moving
bacteriophage viruses and that the parasitic virusesrepresent a later adaptation when
higher forms of life had come into existence.
To the ancient myths
of creation biologists have added their findings upon the origins
of life. It revealed itself somewhere in the warm, marginal waters of a primeval sea,
so rich in nucleic acids, carbon dioxide, potassium, calcium, sulphur and
phosphorus; in • due course were born unicellular and defenceless organisms
sensitive to light which, belonging to neither the animal nor the vegetable kingdom,
fed upon inorganic matter. In the course of time, perhaps due to intensification of
sunlight or to cosmic radiation, mutant molecules appeared and the organisms
developed into the first single-celled plants.
A blue-green alga is
generally accepted as representative of the first vegetal life.
Although lacking a true cell nucleus it is impregnated with the green substance
chlorophyll, which means that it is capable of photosynthesis—that is, of
transforming, with the aid of light, atmospheric carbon dioxide and hydrogen from
water into carbohydrates. This change is accompanied by a release of oxygen—a
vital factor, for by releasing free oxygen into an atmosphere previously deficient in it
the process of photosynthesis slowly created conditions essential to the
development of a varied flora, and subsequently of marine and terrestrial fauna
dependent in turn on available oxygen and abundant plant life for food. The text books
say it was about fifteen hundred to two thousand million years ago that the oceans
were invaded by algae, and indications of organic matter have been found in rocks
over two thousand million years old in southern Africa. But these conclusions may
have to be reconsidered: in 1967 a Swedish chemist discovered that an alga of the
type known as Chlorella, taken from the mud of stagnant water, is capable when
grown in a strictly controlled atmosphere of 100 per cent methane of producing by
photosynthesis an atmosphere containing 6 per cent oxygen in a matter of days.
Such an isolated experiment cannot be considered as proof, but it raises the
question of the possibility of the existence of such forms of life, and of an
atmosphere containing free oxygen, at an earlier geological period than is generally
considered probable.
The first traces of
primitive life have definitely been identified in rocks of Pre-
Cambrian date. By the dawn of the Cambrian period invertebrate sea creatures with
hard 'shells' had come into existence, as fossil finds testify. We are justified in being
astonished at the great
diversity and complexity of flora and fauna which have
developed from single- celled forms in the teeming seas since the transition to the
Palaeozoic era a mere five hundred or so million years ago.
At first this life
was passive, at the mercy of the waters. Later the organisms grouped
themselves in colonies, became many-celled, acquired a digestive system and the
organs required for breathing, motion, reproduction; they learned to 'swim' and to
move direction-ally. The microscopic algae gave birth to dense seaweeds and water
grass; the minute animal forms gave rise to echinoderms such as sea urchins,
arthropods, molluscs. In manuals of natural history this chapter of evolution seems to
follow a simple and logical sequence, but the complex evolutions of which the
Cambrian seas were the theatre are among the obscurest secrets of the world's
history. Over half of the forty divisions of animal life, as now classified, have the sea
as their only habitat. The majority of the remainder are represented in the sea, in
fresh water and on land. Barely a tenth have broken all contact with the liquid
element. Marine existence embraces a great variety of forms, adapted to surface
waters or deep sea, reefs or strands, rock pools or brackish shallows; in the water
there is room for all.
The basis of every
higher form of marine life is phytoplankton, which drifts in the
uppermost waters of the ocean. These single-celled plants 'feed' on the mineral salts
in sea water by means of photosynthesis and form enormous trails, especially in
cold seas. Phytoplankton may be said to play the part in marine life that grass plays
in the economy of dry land farming. In the spring, when the longer hours of daylight
warm the surface waters, the plant plankton flourishes and sailors say that the sea
'blooms', and it is a fact that the ocean is touched with green over vast expanses.
The cycle of the phytoplankton is complicated, but broadly speaking the spring
blooming depletes the mineral salts available and thus the plankton declines as
autumn approaches. Upwellings and currents due to temperature differences restore
the level of nutrient salts in the surface waters during the cold days of shorter
sunlight which follow, and lead again to abundance of plankton in the spring. Drifting
with the plant plankton and living on them are the zoo-plankton, microscopic animal
forms which in turn provide food for larger kinds of marine fauna, and so on up the
scale to the fish we ourselves eat and to the marine mammals.
Plant and animal plankton
take many different forms. Among the phytoplankton the
diatoms, the most numerous, form from the sea's minerals an external cell wall of
silica. The microscope reveals them in astonishing variety—pillbox shape,
rectangular, irregular; other kinds form chains or spheres, are star-shaped, anchor-
shaped, have whip-like flagella. The zooplankton, Radiolaria and Foramini-fera for
example, are no less varied and strange. Included among the zooplankton are fish
eggs and fish of various kinds in their early stages of growth, when they too must drift
with the currents, unable yet to swim where they will.
At different periods
in the Earth's past billions of planktonic plants and animals lived
and died, and their remains sank to form sediments in ancient seas, there to undergo
a chemical and physical transformation due to pressure. Over millions of years of
geological change these viscous deposits became trapped under layers of
impervious rock. Today forests of derricks rise in those places where petroleum was
thus created.
Plankton has had a
further share in the Earth's geological formation. The remains of
planktonic plants and animals lie in thick layers on the floors of our oceans. The 'star
performer' is unquestionably a foraminifer known as Globigerina bulloides; some
thirty- five million squares miles of sea bed are covered with a mud called the
Globigerina Ooze, which, if the findings of the Swedish research ship Albatross are
accurate, in places reaches a thickness of some twelve thousand feet.
Plankton allows us
to imagine what life was like in early Palaeozoic seas: vast
masses of algae, now expanding, now receding, and with them a fantastic pullulation
of animalcula. It is true that zooplankton makes up hardly more than two or three per
cent of the whole, but precise calculations have established that over a thousand
million tons of a single species, Euphasia superba, are born and grow each year in
the Polar seas.
By the end of the Cambrian
period, about four hundred and twenty million years ago,
all the main divisions of the invertebrates were represented in the seas. Cambrian
rocks are well exemplified in the British Isles (the term Cambrian comes from the
Roman name for Wales). Britain and most of Europe lay beneath an ocean which
has been named Poseidon—roughly a more extensive Atlantic, although the seas
were at that time retreating. In Poseidon's waters were molluscs, worms, starfish,
sponges, and especially a group of arthropods called trilobites, the commonest fossil
representative of that age.
